This invention relates generally to air-core transformers, and more particularly, to a planar air-core transformer design to replace a traditional parallel resonant balun that is comprised of a co-axial inductor and capacitor that is used in receiver coils in magnetic resonance imaging (MRI) systems to isolate coil elements. The transformer structure provides enhanced isolation at a specific frequency that can be used to replace a traditional transformer or to reduce common mode currents when used in high frequency switching power electronic converters and thereby reducing EMI generation and subsequently filtering requirements.
FIG. 1 illustrates a traditional balun 10 for MRI receiver coils that can be used multiple times, depending upon the location of the corresponding coil cable bundle exiting the corresponding structure, relative to the receive coil element. Balun 10 includes a common mode inductor 12 connected to a co-axial cable 14 at each end. Balun 10 further includes a common mode capacitor 16 external to a copper shield 18 that surrounds and shields the common mode inductor 12. External copper shield 18 includes an end cap 20 at each end to fully encapsulate the common mode inductor 12 within its shielded environment. A hole (not shown) may be centered on one of the end caps to allow for a brass tuning screw in some embodiments. Traditional balun 10 is used to isolate coil elements in MRI system receiver coils.
Optical isolating devices are generally used to provide signal isolation in power converters such as that depicted in FIG. 2 that illustrates a transformer employed in a medium voltage power electronic converter. Compact transformers are used in rare instances, but are limited to medium voltage (<1500V) systems.
Ethernet transformers have been designed to provide common mode isolation with differential mode matching to ensure the best possible transmission of the data signals. FIG. 3 illustrates an equivalent circuit for one Ethernet transformer that is known in the art. These transformers are limited in the isolation voltage that they can provide.
It would be desirable to provide a transformer that lends itself for integration in printed circuits, e.g., flexible printed, PCB, etc., and that provides higher isolation than traditional signal transformers at a particular and useful frequency such that the transformer is suitable to replace a traditional parallel resonant balun that is comprised of a co-axial inductor and capacitor such as the one depicted in FIG. 1 or a simple transformer used for power supply isolation or data isolation in high frequency switching power electronic converters.